1. Field of the Invention
The present invention relates to a high efficiency linear power amplifier of plural frequency bands capable of using two or more plural frequency bands by changing over, and a high efficiency power amplifier used in cellular telephone, PHS and other mobile communication appliances.
2. Related Art of the Invention
Hitherto, as a high efficiency linear power amplifier of plural frequency bands, the constitution as shown in a block diagram in FIG. 7 has been known. In FIG. 7, in an input terminal 20 of frequency band A, for example, a signal of 950 MHz band (940 to 956 MHz) is entered, matched in impedance in a matching network 21, amplified in a pre-amplifier 22, matched in impedance in a matching network 23, amplified in a post-amplifier 24, matched in impedance in a matching network 25, and obtained as signal output of frequency band A of 940 to 956 MHz from an output terminal 26.
Similarly, in an input terminal 27 of frequency band B, for example, a signal of 1,900 MHz band (1895.15 to 1917.95 MHz) is entered, matched in impedance in a matching network 28, amplified in a pre-amplifier 29, matched in impedance in a matching network 30, amplified in a post-amplifier 31, matched in impedance in a matching network 32, and obtained as signal output of frequency band B of 1895.15 to 1917.95 MHz from an output terminal 33.
In such conventional high efficiency linear power amplifier of plural frequency bands, circuits of similar constitution are required in both frequency band A and frequency band B, and the number of parts is great and the occupation space is significant. Accordingly, as shown in FIG. 8, it is attempted to enter a signal of band A, that is, 940 to 956 MHz, or a signal of band B, that is, 1895.15 to 1917.95 MHz in an input terminal 34, adjust a wide band matching network 35 to match the impedance at frequencies in both frequency bands, amplify both frequency bands in a wide band pre-amplifier 36, match the impedance similarly in both frequency bands A and B in a wide band matching network 37, amplify in a wide band post-amplifier 38, match the impedance again in both frequency bands in a wide band matching network 39, change over the frequency bands in a switch circuit 40, and change over and issue to either an output terminal 41 for band A or an output terminal 42 for band B.
However, in this method, although the number of parts may be decreased, the problem is that the adjustment of the wide band matching network (in particular at the later stage) cannot be extremely improved in efficiency. FIG. 9 is an example of output impedance and load impedance of FET of the post-amplifier 38. Generally, the load impedance maximum in gain is a complex conjugate with the output impedance of the FET. Incidentally, the load impedance maximum in gain, the load impedance maximum in efficiency, and load impedance minimum in distortion differ in each frequency band.
Generally, in the post-amplifier 38, the emphasis is placed on low distortion or high efficiency, and in the wide band matching network 39, therefore, it is necessary to compose the matching network to realize such characteristics, but it is hard to design and the characteristics are likely to deteriorate because plural restraint conditions are present in plural frequency bands. In a switch circuit 40 using an electronic circuit, meanwhile, since the passing electric power is large, the elements to be used must have a large capacity, and the loss is large because it is used in a place of high power level, and the efficiency of the power amplifier cannot be enhanced.
Recently, cellular telephones are rapidly promoted in the trend of smaller size, lighter weight and lower cost. Accordingly, owing to the restriction of the battery used in the cellular telephones, the power amplifier is required to have both high efficiency and low voltage operation.
FIG. 19 shows an example of a constitution of a conventional power amplifier. In FIG. 19, reference numeral 194 is an input terminal, 195 is an output terminal, 196 is a transistor, 197 is a gate bias terminal, 198 is a drain bias terminal 199 is an input matching network, 200 is an output matching network, and 201, 202 are first and second DC blocking capacitors.
In thus constituted power amplifier, a conventional method of enhancing the efficiency is described below.
In the conventional power amplifier, a high efficiency is realized by adjusting the output matching network 200 connected to the transistor 196 so that the load impedance of the transistor 196 may have the maximum efficiency at a desired output power.
In this constitution, however, when the output power is lower than the specified output power, the efficiency drops as shown in FIG. 20.